1. Field of the Invention
This invention relates to a method of forming a pyrolytic tin oxide coating on a face of a hot glass substrate during transport of the glass through a coating station into which a coating precursor solution containing a tin compound is sprayed so that the glass is contacted by material from which the tin oxide coating is formed by pyrolysis. The invention also includes flat glass bearing a pyrolytic tin oxide coating.
2. Description of the Related Art
For many applications, for example for window glazings, the coating should be colourless, or it should at least have a colour which is aesthetically acceptable. Because the optical thicknesses of such coatings as may be used are comparable with the wavelength of light, the coatings tend to be coloured in reflection due to interference effects. Such interference effects tend to be more pronounced in relatively thin coatings. It has been found that coatings which exhibit a slight blue or green coloration are much more acceptable commercially than those exhibiting other colours.
It is well known to provide tin oxide coated glass. Tin oxide coatings may be rendered conductive so that the coating reduces the emissivity of the coated glass in respect of long wavelength infra-red radiation, in particular radiation having wavelengths greater than 3 micrometers.
It is known to render tin oxide coatings conductive by incorporating doping agents, and they may also include minor proportions of other compatible materials for various purposes. The nature and amount of any atoms present other than tin and oxygen should not exceed a limit above which the crystal lattice structure type of the coating differs from that of cassiterite, so as to preserve the transparency and durability of the coating. A simple, and perhaps simplistic, explanation of doping is that atoms are provided which are compatible with the tin oxide crystal lattice, and which have a different valency shell from both tin and oxygen. As a result, the doping atoms provide spare electrons, or electron gaps which can act as charge carriers through the coating.
The most common doping agent is fluorine which can replace oxygen. Fluorine has 7 electrons in its valency shell while oxygen has 6. An alternative doping agent which has been proposed is antimony. It should be noted however that antimony is known to have a strong colouring effect on tin oxide coatings, so it is not normally used as a doping agent in coatings of transparent glazings, especially when a high total luminous transmission is required.
It should be borne in mind that the tin oxide coating will rarely be stoichiometrically pure tin dioxide. There are almost certain to be some tin atoms in the lower valency state and some unfilled oxygen positions in the tin oxide lattice. In fact it has been noted that at high temperatures a tin oxide coating may be conductive even without a doping agent. It may be that a doping agent, when present, combines in some way with such oxygen gaps in order to achieve conductivity. In any event, the present invention does not depend for its usefulness on any theory of the doping mechanism.
Such coated glass is often used for glazing purposes to provide a measure of heat conservation, and also to provide a heat screen, for example a solar screen. Most solar radiation energy is at relatively short wavelengths, so that it can be transmitted by the coated glass provided that the coating and the glass are clear, but radiant energy from the interior of the glazed structure tends to be at longer wavelengths, so it is inhibited from escaping from the structure through the coated glazing. Such coatings are often made to a thickness in the range 200 nm to 800 nm.
It is known that such coatings should desirably fulfill certain criteria.
The reduction in emissivity should be substantial in order that the heat gain should be economically worthwhile having regard to the additional cost involved in coating the glass. This tends to imply a rather thick coating in order that the necessary conductivity in the coating can be achieved.
The coated glass should be priced at a level which allows such savings, so it should not be too expensive to manufacture.
The coating should be transparent, that is of low haze, and any haze that is present should be uniform over the extent of the coating. This is relatively unimportant in the case for example of greenhouse glazings, but is quite important for glazings for dwellings and is extremely important in the case of vehicle windows in order to allow clear and uniformly clear vision through the coated glass. Haze, the visible aspect of diffuse light transmission, may be due to surface rugosity of the coating, but this can be cured by polishing the coating. Haze may also be due to internal defects of the coating, whether at the coating/glass interface, or within the thickness of the coating. It will be appreciated that such internal haze tends to be greater, the greater is the thickness of the coating. The requirement for low haze is therefore at odds with the requirement for low emissivity.
There are various coated glass products on the market.
One such product comprises float glass on which a tin oxide coating some 750 nm to 800 nm in thickness has been formed pyrolytically. This coating has excellent low emissivity, less than 0.2. Such low emissivity is in fact as good as can be achieved by applying a coating by a sputtering technique. The coating also has good colour in reflection, in that it is a barely perceptible green. But because of its thickness, and also due to formation of the coating by pyrolysis, this coating has a level of haze which, while it is commercially acceptable for many purposes, is not as good as it could be. Some contrast in the haze over the extent of the coating area may also be apparent on inspection. When this coating is polished so as substantially to eliminate surface haze, any residual haze may be attributed to defects below the surface of the coating. This residual haze is referred to herein as internal haze. This known coating has an average internal haze value of 2%.
References to "internal haze" throughout this specification are references to internal haze measured according to American Standard ASTM D 1003-161. References to "emissivity" throughout this specification are references to normal emissivity as defined in Section 5.1.1. of Belgian Standard NBN N 62-004 (1987).
It will be appreciated that one would expect the internal haze of such a coating to increase with increase in its thickness, so comparing actual haze values can be misleading. A more direct comparison can be made by dividing the percentage haze value by the thickness of the coating expressed in micrometers to give a factor for specific internal haze. If this is done, it will be noted that the previously known coating has a specific internal haze factor of more than 2.5. Specific internal haze factors of more than 2.5 are typical of known pyrolytic tin oxide coatings.